The present invention relates to a method and device for verifying proper alignment of a tool attached to a robotic arm. More particularly, the device and method allow teachable, automatic actions of a robotic welder to determine whether the welding electrode is properly positioned relative to the robot prior to welding. If there is misalignment for any reason, the automatic cycle can be halted and an operator alerted to identify the source. In short, the method and device address a problem that is generally referred to as "torch alignment" or "tool alignment" verification.
A brief discussion of a typical robotic welding system will be helpful in understanding the setting in which tool or torch alignments must be verified. A robotic welding system typically repeats a previously taught sequence of robot motions to deliver the welding torch to various seams on the work piece to be welded. The system inherently relies on the positional integrity of the robot motions, the work piece location, the torch location relative to the robot, and the consistency of the shape and alignment of the electrode wire. If too much unpredicted position deviation occurs in any component, the integrity of the delivered weld is jeopardized.
As a practical matter, torch misalignments occur frequently in robotic welding applications. Misalignments can be caused by a torch collision resulting in deviations in the torch or linkages, a robot collision resulting in deviations in the robot axes, deviations due to the replacement of consumables in the torch, mispositioning of the torch or linkages during maintenance activities, problems with the "cast" or "helix" of the wire electrode which cause it to bend unpredictable as it is fed from the torch (incoming wire stock or the wire feed system), deterioration of the torch consumables to the point where they cannot repeatably direct the wire electrode, and any other inadvertent shifting of the robot relative to the work piece as a result of a collision or maintenance activity. Welding with a misaligned torch can potentially damage the robot, the welding gear, and the work piece. Sometimes the damage to the work piece cannot be detected by the outward appearance of the weld. Many work pieces may be inadvertently produced with bad welds before the problem is detected.
Two widely used methods of tool or torch alignment are:
1. periodic manual intervention by the operator using a jig to check tool alignment, and PA1 2. automatic execution of unique brand-specific robot algorithms relying on software and special robot features to detect and correct tool misalignments.
The manual intervention method used to detect and correct electrode misalignment involves manually stopping the automatic cycle of the robot and removing the torch nozzle from the body of the torch to expose the tip holder located thereunder. With the cap-like nozzle removed, the welding tip conventionally attached to the top holder can be removed and replaced with an alignment tool. Then, the alignment tool is positioned into an alignment jig that has been placed in a known position within the work envelope of the robot. Linkage adjustments are then made to match the robot arm to the gauge dimension and snugly fit the alignment tool in a bore or sliding sleeve on the alignment jig. Thus, the sliding sleeve is used as a go/no-go gauge to check the tool alignment and orientation with respect to the torch.
This manual intervention method has several shortcomings. First, deviations resulting from cast or helix conditions of the wire electrode are not detected since the wire electrode is removed for the alignment check. Furthermore, this method relies too heavily on the discipline of the operator to periodically interrupt the cycle and perform the alignment check. Finally, this method may result in a significant time delay between the occurrence and detection of an alignment problem, potentially allowing many pieces to be welded incorrectly in the interim. Often, the faulty welds are difficult, if not impossible, to detect visually.
The second common method of torch or tool alignment verification relies on a "touch sensing" feature which some robot manufacturers build into their robots. Robots so equipped can perform a "touch sense" on a gauge block placed in a known location. The robots know the exact location of the gauge block and use the torch or electrode to electrically detect the actual position of the gauge block relative to the torch. Since the exact position of the gauge block is known and fixed, all detected error is attributed to torch or tool position deviations. Using this information, the robot mathematically compensates for any misalignment by establishing a new theoretical tool point to control the path of the robot arm.
This touch sense and internal compensation method has several shortcomings. The method cannot distinguish between torch and robot alignment problems. If one or more of the robot axes is misaligned due to a crash, the robot will incorrectly attribute the error entirely to the torch. This method may also incorrectly adjust a tool point when the real problem is that the condition of the consumables is causing a lack of repeatability. Furthermore, when the robot attempts to compensate for substantial torch misalignments, the robot arm may encroach physical constraints at other points in the work envelope, thus causing serious damage to the arm itself as well as other equipment within the work envelope.
Therefore, a primary object of the present invention is the provision of an improved method and device for verifying proper alignment of the tool held by the arm of a robot.
A further object of the present invention is the provision of a device that detects both misalignment and misregistration of the tool with respect to its expected position.
A further object of the present invention is the provision of a method of verifying proper tool alignment while the tool is in the holder, thus avoiding having to manually remove the tool or disassemble and reassemble the tool assembly.
A further object of the present invention is the provision of a method that is teachable to the robot so as to facilitate the high quality, automatic production with the robot.
A further object of the present invention is the provision of a device and method that will signal misalignment of the tool from any cause.
A further object of the present invention is the provision of a device for verifying proper tool alignment that is adaptable to almost any kind of conventional robot.
A further object of the present invention is the provision of an alignment checking device that can be easily retrofitted to any existing robotic welding application.
A further object of the present invention is the provision of a device that can be mounted on or near the work piece such that the robot is taught to automatically check the alignment of an arc welding electrode in conjunction with the cycle for each part.
A further object of the present invention is the provision of a method that is particularly well-adapted to detecting dent, misaligned, or mislocated electrodes in a robotic welding system.
A further object of the present invention is the provision of an improved device for verifying alignment of robotically held tools that is economical to manufacture and durable in use.
These and other objects will become apparent from the following description of the invention.